(1) Field of the Invention
This invention relates to a substrate treating apparatus for treating semiconductor wafers, substrates for liquid crystal displays, substrates for plasma displays, substrates for organic EL, substrates for FEDs (Field Emission Displays), substrates for optical displays, substrates for magnetic disks, substrates for magnetic optical disks, substrates for photomasks, substrates for solar cells and so on (hereinafter simply called substrates). More particularly, the invention relates a technique of treating substrates by immersing the substrates in a phosphoric acid aqueous solution.
(2) Description of the Related Art
Conventionally, this type of apparatus includes a treating tank, a pump, a heater and a filter which constitute a circulating line. The treating tank stores a phosphoric acid aqueous solution, and substrates are immersed in the phosphoric acid aqueous solution for treatment. The phosphoric acid aqueous solution discharged from the treating tank is returned to the treating tank again via the pump, heater and filter. A silicon concentration meter is mounted on this circulating line. The silicon concentration meter measures silicon concentration by detecting absorbance of a specific wavelength based on light transmitted through the phosphoric acid aqueous solution (as disclosed in Japanese Unexamined Patent Publication No. 2008-103678, for example).
The substrate treating apparatus constructed in this way can measure silicon concentration in the phosphoric acid aqueous solution flowing through the circulating line. Based on the measurement result, the silicon concentration in the phosphoric acid aqueous solution can be adjusted. Adjustment of the silicon concentration will be described. An example is here taken of a treatment of substrates having silicon oxide film and silicon nitride film formed thereon, in which the silicon nitride film is selectively etched, leaving the silicon oxide film on the substrates. In this case, the higher silicon concentration will result in the lower etching rate of the silicon nitride film, and the greater increase in the quantity of particles in the phosphoric acid aqueous solution, thereby shortening the life of the phosphoric acid aqueous solution. On the other hand, the lower silicon concentration will result in even the silicon oxide film etched more than is needed (i.e. will lower etching selectivity for the silicon nitride film). Therefore, the silicon concentration is adjusted within a predetermined range for inhibiting lowering of the etching selectivity while inhibiting lowering of the etching rate of the silicon nitride film.
However, the conventional example with such construction has the following problems.
The conventional apparatus, since the silicon concentration is estimated from the absorbance of the phosphoric acid aqueous solution, has an inconvenience that the accuracy of the measurement result of the silicon concentration is not so high. Since absorbance is easily influenced by the temperature of the phosphoric acid aqueous solution, there is an inconvenience that large temperature changes of the phosphoric acid aqueous solution render the accuracy of the measurement result unstable (variable).